In a mobile radio system that comprises a base station with two antennas and a mobile radio subscriber, there is a fixed phase relationship between the two transmission channels, which can respectively be associated with one of the antennas of the base station. In the UMTS (Universal Mobile Telecommunications System) Standard, the relationship between the signals emitted from the two antennas can be influenced by means of a so-called CLTD (Closed Loop Mode Transmit Diversity) function in order in this way to achieve constructive interference between the two transmission channels. The CLTD function can be operated in a mode 1 and in a mode 2. In mode 1, the phase of one of the two antennas can be varied, while the phase of the other antenna remains fixed. Overall, this results in the phase relationship between the two antennas being varied. In addition to the variation of the phase relationship known from mode 1, mode 2 provides for variation of the amplitudes of the signals emitted from the antennas. The amplitudes of both antennas can be varied in mode 2.
The CLTD function is described in the UMTS Specification 3GPP TS 25.214. This specification, in particular Section 7 and Annex A, is hereby incorporated by reference in the present application. This specification is referred to repeatedly in the following text. All the details in this case relate to Version V5.6.0 (2003-09).
The CLTD function allows FSM (Feedback Signalling Message) data words to be formed from estimated channel impulse responses in the mobile radio, and these are transmitted to the base station. The channel impulse responses of both channels are always used for an FSM data word. The FSM data words contain information intended for the base station about the optimum phase relationship and, possibly, about the optimum amplitudes of the signals to be transmitted from the two antennas.
Two antennas are used for transmission in a base station for the transmission diversity method considered here. The base station forms two antenna weights w1 and w2 from one FSM data word, and these are applied to the signals that are intended to be emitted from the two antennas. This is illustrated in FIG. 1. In order to avoid the possibility of destructive interference between the signals from the two antennas, the payload data (DPDCH) and the so-called dedicated control data (DPCCH) in both transmission paths are multiplied by the complex weighting factors w1 and w2. The dedicated control data contains training symbols, which are referred to in the following text as dedicated pilots. In parallel with this, the so-called CPICH symbols or common pilots are transmitted as training data, and these are not multiplied by the weighting factors. Thanks to the orthogonal characteristics of the CPICH symbols, the channel parameters h1,i(p) and h2,i(p) relating to the two transmission antennas can be determined in the receiver. In contrast, the channel parameters estimated on the basis of the dedicated pilots are denoted h1,i(d) and h2,i(d). In the case of mode 2, it is only possible to estimate the resultant channel for the two antennas on the basis of the dedicated pilots, and its channel parameters are denoted hid in the following text.
The antenna weights are calculated in the base station on the basis of information that is sent back from the mobile part. The respective optimum transmission weights are determined in the mobile part by evaluation of the OPICH symbols, and are quantized using a rule which is known in the base station, are mapped onto a bit sequence, and are transmitted to the base station. In this case, one bit (the so-called FBI (feedback information) bit) is in each case transmitted for each UMTS time slot and the weights in the base station are then adapted. In the case of mode 1, the current weights are determined by a vector with two bits {b1, b2}, and they are thus dependent on the current transmitted bit and on a previous bit. In the case of mode 2, the current weights are determined by a vector with four bits {b1, b2, b3, b4}, that is to say in this case the memory is four bits. These transmission weights must be taken into account during the demodulation of the received data in the mobile part. The previously determined, quantized weights may be used in this case. Channel influences can result in transmission errors during the transmission of the FBI bits to the base station. This leads to the weights assumed for the demodulation in the mobile part differing from the actually used transmission weights, and this leads to serious errors in the demodulation of the data. Simulations show that the receiver performance required by the Standard cannot be achieved in this way. In order to avoid this, so-called antenna verification is used in the receiver to check whether the antenna weights being used in the transmitter actually match the path weights calculated in the mobile part. In the event of a discrepancy, the weights are appropriately corrected.
Methods for antenna verification for both mode 1 and mode 2 are proposed in Annex A of the Standard mentioned initially. A test of two hypotheses is proposed for mode 1, specifically based on the inequality:
                              2          ⁢                                    ∑                              i                =                1                            Npath                        ⁢                                                  ⁢                                          1                                  σ                  i                  2                                            ⁢                              {                                                      2                                    ⁢                                      Re                    ⁡                                          (                                              γ                        ⁢                                                                                                  ⁢                                                  h                                                      2                            ,                            i                                                                                (                            d                            )                                                                          ⁢                                                  h                                                      2                            ,                            i                                                                                                              (                              p                              )                                                        *                                                                                              )                                                                      }                                                    >                  ln          ⁡                      (                                                            p                  _                                ⁡                                  (                                                            ϕ                      Rx                                        =                    π                                    )                                                                              p                  _                                ⁡                                  (                                                            ϕ                      Rx                                        =                    0                                    )                                                      )                                              (        1        )            for even-numbered UMTS time slots and
                                          -            2                    ⁢                                    ∑                              i                =                1                                            N                path                                      ⁢                                                  ⁢                                          1                                  σ                  i                  2                                            ⁢                              {                                                      2                                    ⁢                                      Im                    ⁡                                          (                                              γ                        ⁢                                                                                                  ⁢                                                  h                                                      2                            ,                            i                                                                                (                            d                            )                                                                          ⁢                                                  h                                                      2                            ,                            i                                                                                                              (                              p                              )                                                        *                                                                                              )                                                                      }                                                    >                  ln          ⁡                      (                                                            p                  _                                ⁡                                  (                                                            ϕ                      Rx                                        =                                          π                      /                      2                                                        )                                                                              p                  _                                ⁡                                  (                                                            ϕ                      Rx                                        =                                                                  -                        π                                            /                      2                                                        )                                                      )                                              (        2        )            for odd-numbered UMTS time slots.
The bit that is actually being used can then be deduced on the basis of the result of this hypothesis test. Since the weight for antenna 1 is constant in mode 1, and the weight for antenna 2 can assume only four discrete values with a large Euclidean distance, the reliability of the verification process described by the formulae (1) and (2) is intrinsically very high.
A test of 16 hypotheses is proposed for mode 2. In this case the maximization:(ŵ1, ŵ2)=arg<maxw1,w2εT{ln({circumflex over (p)}(w1, w2))+ln( p(w1,w2))}>   (3)is carried out, where p(w1,w2) is the a-priori probability of the occurrence of a weight pair (w1, w2) This can be calculated from the (known) transmitted FBI bit and from a hypothesis for the error probability during the transmission of the FBI bits. The probability of the presence of a specific weight pair that can be determined on the basis of the training symbols is calculated using
                              ln          ⁡                      (                                          p                ^                            ⁡                              (                                                      w                    1                                    ,                                      w                    2                                                  )                                      )                          =                  -                      (                                          ∑                                  i                  =                  1                                                  N                  path                                            ⁢                                                          ⁢                                                                                                            h                      i                                              (                        d                        )                                                              -                                                                  γ                        (                                                                                                            w                              1                                                        ⁢                                                          h                                                              1                                ,                                i                                                                                            (                                p                                )                                                                                                              +                                                                                    w                              2                                                        ⁢                                                          h                                                              2                                ,                                i                                                                                            (                                p                                )                                                                                                                                                                                2                                                        )                                                                      σ                    i                    2                                    ⁡                                      (                                          1                      +                                                                        γ                          2                                                ⁡                                                  (                                                                                                                                                                                      w                                  1                                                                                                                            2                                                        +                                                                                                                                                            w                                  2                                                                                                                            2                                                                                )                                                                                      )                                                                        )                                              (        4        )            where hi(d) are the channel parameters determined on the basis of the dedicated pilot channel for the resultant channel of the two antennas, h1,i(p) and h2,i(p) are the channel parameters for the antennas 1 and 2 determined on the basis of the common pilot channel, γ2 is the ratio between the signal-to-interference-and-noise (SINR) values for the DPCH pilot channel and for the CPICH channel, Npath is the number of propagation paths, and σ22 is the sum of the noise and interference power on the i-th propagation path.
This procedure involves the two following disadvantages. The decision metrics used are based only on the currently received training symbols. Since both weights are modified in the case of mode 2 and the constellations of possible weight factors have considerably shorter Euclidean distances than in the case of mode 1, there is a major risk of the weights being determined incorrectly. Particularly in the case of mode 2, in which the phase differences between the weights in question are smaller, and in the case of a metric which is based on only one received symbol, the risk of incorrect detection is relatively high. Furthermore, in the case of mode 2, the complexity of the solution is also very high, since 16 metrics must be calculated using equation (4), with the calculation of each individual metric on its own intrinsically being considerably more complex than in the case of mode 1.